1. Field of the Invention
The present invention is directed to a solar cell having a front surface and a rear surface, in which a first p-n-junction is provided in the substrate close to a front surface to separate the substrate into a front portion having a first doping and a rear portion having a second doping. A front layer including a further p-n-junction is provided on the front surface of the substrate to separate the front layer into a front portion having the first doping and a rear portion having the second doping and wherein said front layer front portion is separated from the substrate front portion. At least one first electric contact is provided on the front side of the solar cell and electrically connected to the front portion of the front layer and at least one second electric contact, which is provided on the rear side of the solar cell is electrically connected to a contact point provided on the front side of the solar cell. It is further directed to a solar cell assembly of such solar cells and to a method of manufacturing a solar cell assembly,
2. Discussion of Background Information
Embodiments of the present invention include a new efficient method to manufacture a solar cell with electrical contacts of both polarities on the front surface, improved design features for a spatial arrangement of a plurality of this type of solar cells into a photovoltaic assembly as well as an automated manufacturing flow possible with this arrangement. The features of these embodiments, which are mainly focused on solar cell and solar cell assemblies for space use, but not necessarily limited to it, will become evident in a comparison with the prior art.
For power generation in space, multi-junction solar cells based on III-V semiconductors with current efficiencies between 25-30% are used. Further efficiency increase is likely to occur with adapted designs in the future.
A cross section through a prior art triple junction (TJ) cell is illustrated in FIG. 1. The vertical dimensions are not to scale and laterally the cell extends further to the right.
In such a triple junction solar cell design, three p-n-junctions 1, 2, 3 are stacked on top of each other, such that the band gap of the semiconductor materials constituting the junctions decreases from the top to the bottom of the cell. A Germanium wafer 4, typically 80-150 μm thick, serves as the substrate of the cell. A p-n-junction 3 is created, some μm from the wafer front surface by diffusion. On top of the wafer, two more p-n-junctions 1, 2 are grown from III-V semiconductor materials. It is implicitly understood here as well as in the following that tunnel junctions are included between each p-n-junction. In addition, several additional layers functioning for example as back surface fields, buffer layers, reflectors are included in the actual device. These however, are not relevant in the frame of this invention and are therefore neither referenced in FIG. 1 nor in the following. The top most cell layer is passivated on the surface by a transparent window layer 5. In this configuration the three p-n-junctions are located within a depth of approximately 10 μm from the cell surface.
The cell shape has then to be isolated from usually circular Germanium wafer and the epitaxially grown junctions on top of it. The cell circumference is defined by etching a so called “mesa” groove 6, sufficiently deep (e.g. 10 μm in this example) to separate all three p-n-junctions. The remaining electrically inactive Germanium wafer is then separated mechanically by dicing, i.e. by sawing along a cut line 7.
Then electrical contacts have to be applied to the cell. The rear side contact 8 is usually completely metalized with any metal system able to form an ohmic contact to the cell. The front side contact 9 is produced similarly. Due to the fact, however, that light has to enter the cell from the front, the front side contact is shaped in the form of narrow grid fingers 100 (FIG. 2), which are connected among each other and finally to a number of front side contact pads 10 used for the external cell connection. A typical layout is illustrated in FIG. 2.
Underneath the front side grid another doped semiconductor layer is present, the so called cap layer 11, primarily to serve as additional protection of the cell during attachment of external electrical connections on the contact pads 10 by methods such as soldering or welding. To minimize light reflections, an antireflection coating 12 is deposited on the cell front side.
It is immediately obvious and can be seen in FIG. 3 that for these epitaxially grown cells 200 the two electrical cell contacts are located on different cell surfaces. For simplicity, the back contact is referred to as the (+) contact in the following, whereas the (−) contact is on the front. Since the output of a single cell, e.g. 0.5 A at 2.3 V for the cells outlined above, is incompatible with the requirements of the power subsystem, these cells have to be connected in series and in parallel to increase the output voltage and current as shown in FIG. 3.
The primary element for solar arrays in space is a connection of typical 30 or 60 cells in series for required bus voltages of 50 or 100 V. Several of these elements, called “strings” are then connected in parallel at their ends with the inclusion of blocking diodes to sections and deliver currents around 10 A. The strings are placed on a common insulating substrate 201. Together with the electrical parallel connection of the strings this forms the photovoltaic assembly (PVA). Solar arrays for space use are for example sandwich panels with a carbon fiber face sheet and an aluminum honeycomb core with a Kapton front side insulation. These substrates have large thermal expansion mismatch relative to the cells. During eclipse phases with temperature fluctuations up to 200° C. the gap 202 between two cells varies by several 10 μm, exposing any electrical connection between two cells to significant cyclic stress. To cope with this, it is state of the art to use 10-20 μm thick metal foils like Silver (Ag), Gold (Au), silver-plated Molybdenum (Mo), silver-plated nickel-cobalt ferrous alloy (e.g. known under the trademark “Kovar” and having a low coefficient of thermal expansion), etc. to bridge the gap between two cells.
With the electrical cell contacts on opposite sides in the prior art embodiments, the electrical series connection naturally has to be performed from the front side contact pad 10 of one cell to the rear side 203 of the adjacent cell. To minimize the stresses induced into the interconnector material due to the thermally induced gap variation, the interconnector 204 is S-shaped, protruding significantly above the cell.
Another important consideration in this stringing operation is directed to reverse voltages. For individual cells in a string operating conditions can occur, where particular cells are exposed to a reverse voltage. III-V multi-junction cells, however, can only tolerate a limited reverse voltage. Each cell thus has to be protected from excessive reverse voltage by a diode of opposite polarity compared to the cell which is connected in parallel.
There are several concepts to achieve this, for example growing a diode of opposite polarity separately on a part of the cell surface. The befit of these diode designs is that they are easily connected, e.g. only a connection to the neighboring front side contact pad has to be made. These diodes protect the cell they are grown on. It will become clear in the following that the cells with this type of diodes do not require any additional diode related consideration in the frame of this invention and are therefore not discussed further.
The drawback of these monolithically grown diodes is that they require several additional growth steps during cell manufacturing, which makes the cell considerably more expensive.
Another diode concept frequently used is much simpler to implement, but more complicated to interconnect. A limited area of the cell 101, as shown in FIG. 2, preferably at the cell edge, is electrically isolated by a mesa groove etch 6, which separates all three junctions. This cell area is thus electrically isolated from the rest of the cell, only contacted to the (+) polarity of the cell. By depositing a metal contact 102 on top of it, a (−) contact is provided. Optionally several of these diode p-n-junctions can be etched away to reduce the diode voltage. Due to the fact that the diode created out of cell material in this fashion has the same polarity as the cell, it cannot protect the cell it is located in. Rather an electrical connection has to be made by another S shaped interconnector 205 from the front side contact of the diode 102 to the rear side 206 of the neighboring cell.
To limit radiation damage of the cells in space, all active cell and diode areas should be shielded. Usually a 100 μm thick cerium doped cover glass 207 is used. These are bonded with transparent silicone adhesive 208 to the cell front side. Another silicone adhesive 209 is used for bonding the cells onto the substrate 201. Based on this photovoltaic assembly (PVA) design it is obvious for persons skilled in the art that the manufacturing flow has to involve different steps of manufacturing subassemblies and is hard to fully automate.
Usually the interconnectors of diode and cell are welded to the cell first. Then the cover glass is bonded onto the cell to form a cover integrated cell (CIC). These steps are usually fully automated. Then the CICs have to be positioned with the cover glass facing downwards onto a suitable welding plate and the diode and power interconnectors have to be aligned on the appropriate cell rear side, which is a cumbersome manual process. Note that diode and power interconnectors go to the rear side of different cells. Then all rear side interconnections are performed, again automatically, e.g. by welding.
All subsequent manufacturing steps are then again performed manually. The string of cells created in this way is turned upside down again, by affixing it to a temporary transport plate. Then it is placed with the cover glass front side onto an adhesive foil, which supports the strings during the lay down process. For lay down, a room temperature curing silicone adhesive is applied onto the cell rear side, for example by screen printing, which assures a well defined adhesive thickness. Within the appropriate window of the adhesive's viscosity time curve, the strings are placed onto the support structure. During curing, pressure is applied onto the cells by enclosing the cell in an appropriate fashion and evacuating this area. After complete curing of the adhesive, the self adhesive foil is removed from the front side of the CICs.
It is obvious that this process is very complicated and time consuming and requires several steps that have to be carried out manually.